Cardiac diseases are one of three major causes of death in many advanced countries. For early-stage diagnosis or progress observation of the cardiac diseases, temporal pressure information on the left atrium or left ventricle is used as an index directly useful in diagnosis. What is referred to as the pressure information on the inside of the heart refers to a differential pressure with respect to an atmospheric pressure and shall be called an intracardiac pressure.
For measuring the intracardiac pressure, an invasive method of inserting a cardiac catheter into the inside of a body is adopted. Information acquired by the catheter includes mainly a blood pressure in the aorta or left ventricle.
As a technology relating to noninvasive cardiac pressure measurement, a technique of inferring the stiffness of the cardiac muscle from an eigen-vibration frequency of the left ventricle, and measuring an intracardiac pressure has been devised. A method described in Non-patent Literature 2 is a technique that approximates the left ventricle to a spherical shell, and estimates the stiffness of the cardiac muscle using a relational expression of an eigen-vibration frequency of the spherical shell, which is filled with a fluid, and the stiffness thereof which is introduced in Non-patent literature 1. When the cardiac muscle is stiff, the eigen-vibration frequency increases. For example, when the intracardiac pressure is high, the myocardial tissue gets tensed, and the eigen-vibration frequency of a heart chamber increases. Further, N-patent Literature 3 makes a proposal on a technique of estimating the intracardiac pressure using a relational expression of the stiffness of the cardiac muscle and the intracardiac pressure which is introduced in Non-patent Literature 4.